Prosthetic limbs
by Chris Woodford. Last updated: May 14, 2020.
Have you ever met anyone who has an artificial limb? Chances are you have—without even knowing it. Modern prostheses (as artificial limbs are properly known) work so effectively and look so convincing that you wouldn't even know someone was wearing one. People with prosthetic legs can often climb stairs, walk, swim, and run as well as though they were using natural limbs, while prosthetic arms and hands have advanced to the point where they give individual control of all five fingers. You might think they're boring or mundane, but prosthetic limbs are among the world's truly great inventions: by boosting independence they boost optimism and both these things dramatically improve people's lives. Let's take a closer look at how these amazing devices work!
Photo: Anything you can do (part 1): Prosthetic limbs help to give people back their independence and challenge stereotypes of "disability." Photo by Greg Mitchell courtesy of US Navy.
Contents
What is a prosthesis?
Photo: Anything you can do (part 2): Military amputees learn to surf at the Naval Medical Center San Diego. Photo by Jake Berenguer courtesy of US Navy.
A prosthesis (also called a prosthetic limb, artificial limb, or limb "replacement"—though natural limbs are irreplaceable) is an artificially made substitute for a limb lost through a congenital defect (present at birth), accident, illness, or wartime injury. A type of prosthesis called a cosmesis is designed to be purely cosmetic and has little or no function; artificial hands often fall into this category. Other prostheses are highly functional and have little or no cosmetic disguise; artificial legs, designed to be covered by pants (trousers), are sometimes little more than metal rods and wires.
Types of prosthesis
In theory, any part of the body—from your ear or nose to your finger or toe—could be replaced by a prosthesis. In practice, there are four common types of prosthetic limb, which replace either a partial or complete loss of an arm or leg:
- Below the knee (BK, transtibial): An prosthetic lower leg attached to an intact upper leg.
- Above the knee (AK, transfemoral): A prosthetic lower and upper leg, including a prosthetic knee.
- Below the elbow (BE, transradial): A prosthetic forearm.
- Above the elbow (AE, transhumeral): A prosthetic lower and upper arm, including a prosthetic elbow.
Photo: A typical transfemoral prosthesis. The white part at the top is the socket: it fits snugly around the patient's residual limb. Photo by Greg Mitchell courtesy of US Navy.
How does a prosthesis work?
A prosthesis consists of a number of different parts. There's the prosthetic limb itself, the socket (the connection or "interface" between the prosthetic limb and the patient's body), the attachment mechanism, and the control system. Let's briefly look at these in turn.
The prosthetic limb
Photo: A pair of prosthetic racing "blades." Athletes who wear these use more conventional prostheses in everyday life. Photo by Sara Wood courtesy of US Army.
The construction of a prosthesis depends largely on the job it
will have to do. While cosmetic hands are concerned purely with
appearance, prosthetic legs are substitutes for a major structural
part of someone's body and have to bear their entire weight.
Typically, a prosthetic arm or leg is made from a strong and durable
but lightweight material such as carbon fiber, covered with foam
padding (for comfort) or flesh-colored
The socket
Photo: The socket is the interface between a patient's residual limb and their prosthesis. Without a good socket, even the best prosthesis is going to be uncomfortable and unusable. Photo by Clay M. Whaley courtesy of US Navy.
The comfort and effectiveness of a prosthesis is largely governed
by how well it fits onto the remaining part of the patient's own
limb, which is called their residual limb
(or sometimes, informally, the "stump"—a term some people understandably find offensive,
though it is, nevertheless, widely used in the medical world). The connecting part of a prosthesis is called the socket and it's carefully molded around a plaster cast taken from the residual limb. The fit of a socket has to be precise or the new limb may damage the residual one, causing discomfort or tissue damage and perhaps
even making it impossible to wear the prosthesis for a time. A
patient's residual limb is likely to change shape and size over months
and years so new sockets will be needed from time to time.
More precisely fitting sockets can now be made by scanning a patient's residual limb
with
The attachment mechanism
Photo: This simple transradial (below-the-elbow) prosthetic arm attaches with a socket, straps, and a shoulder harness. Note the relatively crude "hand," which has been specifically designed to help the man do his job. Photo by courtesy of NASA Marshall Space Flight Center (NASA-MSFC).
A well-fitting prosthesis is usually secured to a residual limb by what's called a suspension system (which might be an elastic sleeve, a suction socket, or old-fashioned straps and harnesses). A snug, secure fitting is vital for comfort—and ensures the limb can be properly controlled. Often the socket is itself a part of the attachment: for example, a prosthetic leg socket may consist of a large hollow plastic casing into which the residual limb is inserted.
The control system
Photo: A prosthetist demonstrates a relatively simple prosthetic arm. Note the shoulder socket at the top (the light-colored plastic), the simple elbow joint, and the pincer-hand at the bottom controlled by cables running down from the shoulder. Photo by Greg Mitchell courtesy of US Navy.
Natural limbs are pulled back and forth by muscles stimulated by
our brains; in much the same way, the simplest functional prostheses
are operated by systems of cables that run through them, doing the
job of the muscles. One common type of prosthetic hand is a pincer,
sometimes worn inside a glove (for cosmetic reasons) that can be
opened or closed by pulling cables attached to the opposite shoulder.
Simple artificial legs work largely through gravity: the wearer
learns to walk in them, through practice, by alternately swinging
them into position and then balancing on them while they move their
unimpaired leg in its turn. More sophisticated prostheses are electrically
controlled and powered by
The most sophisticated (and
expensive) prostheses are myoelectric. They use electrodes to
sense muscular impulses in the residual limb (or elsewhere on the
patient's body); Photo: A much more sophisticated prosthetic arm and hand. This ultra-sophisticated i-limb™, made by
Touch Bionics (now a part of Össur), has five working fingers that give good enough grip to pick up marbles. Photo by Charmain Z. Brackett courtesy of US Army. Technology is only part of the story, of course. Getting used to a brand
new part of your body is a physical and psychological challenge involving a partnership
between the patient and their prosthetist. This hugely important
person works with you at every stage of the process, from choosing
the best prosthesis for your needs; through the process of measuring,
manufacture, and fitting; to helping you learn how to operate your
new limb; and tackle new challenges such as taking up a sport or activity.
Learning to live with a new limb
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